It has long been desirable to provide a simple and efficient manner of measuring the content of a low-molecular compound, such as glucose or cholesterol, in a complex medium, such as blood. The present invention has provided such a method and apparatus.
The determination of the concentration of the low-molecular compounds is carried out in connection with chemical reacting the dialysate with an enzyme, e.g. oxidation or hydrolysis, with formation of one or more readily analysable compounds. The measurement can be facilitated in some cases if the dialysate is diluted with a solution of reagent and/or a suitable buffer before the analysis is carried out.
The method in accordance with the invention in the preferred embodiment is intended to be applied to the measurement of glucose concentration in the blood of a patient, whereby the blood can be taken from a patient and introduced into a dialyzer and preferably be returned subsequently to the patient.
In such an applicant the dialyzer used is appropriately a "fibre-kidney" with preferably only one or a small number of fibres around which the blood is connected whilst low-molecular compounds are transmitted to the dialysis fluid passing through, whereby the dialysate is formed which is to be measured. It is appropriate for the dialysis flow through each individual fibre to be controlled separately. As a result the flow through one fibre will be independent of the flow through another fibre. This is achieved appropriately in that each fibre is given a separate inlet and/or separate outlet and is connected to a separate pumping mechanism.
It will be clear to those versed in the art, that the method in accordance with the invention can of course also be applied to quantitative and/or qualitative determinations of low-molecular compounds in complex media other than blood, e.g. in micro-biological cultivating chambers.
In the method described above the low-molecular compounds diffuse over from the complex medium to the dialysis solution in the direction towards a higher pressure. If the dialysis is carried out during a short specified time, the result will be that only a very small part of the low-molecular compounds is removed from the complex medium. The concentration of the low-molecular compounds obtained in the dialysate will be much lower than in the complex medium. The concentration of the low-molecular compounds in the dialysate will, however, in practice be directly proportional to the concentration in the complex medium.
For the measurement of glucose in blood the dialysate is brought into contact with an enzyme, preferably glucose oxidase, appropriately immobilized on a solid matrix, e.g. porous glass. Glucose oxidase catalyzes the oxidation of glucose with consumption of oxygen, formation of hydrogen peroxide and liberation of propones. The difference in the concentration of O.sub.2, H.sup.+ or H.sub.2 O.sub.2, which can be easily determined before and after enzymatic reaction respectively by means of suitable electrodes or photometrically using e.g. a pH indicator, thus gives an answer which, owing to the proportionality, directly indicates the concentration of glucose in the complex medium.
The result of the measurement is appropriately calculator-processed directly so that the evaluated result can be used for treatment of the complex medium, e.g. blood, on which the measurement is carried out.
For the measurement of the glucose concentrations the enzyme hexokinase may also be used as an enzyme together with adenosine triphosphate (ATP), when glucose-6-phosphate and adenosine diphosphate (ADP) are formed with simultaneous changes, e.g. generation of heat, which can be measured. Similarly it is quite possible of course to allow the glucose to participate in an electrochemical process, e.g. in a fuel cell with glucose and oxygen as reactants and obtain by this route a quantitative measure of the glucose concentration.
If the method in accordance with the invention is applied to the measurement of penicillin, penicillinase is suitable for use as the enzyme, the change in the pH which occurs being measured.
If the method in accordance with the invention is applied to the measurement of cholesterol, cholesterol oxidase is suitable for use as the enzyme, the change in the concentration of O.sub.2 and/or H.sub.2 O.sub.2 being measured.
If the method in accordance with the invention is applied to the measurement of uric acid, uricase is suitable for use as the enzyme, the change in the value of the pH being measured.
If the method in accordance with the invention is applied to the measurement of urea, urease is suitable for use as the enzyme and the NH.sub.3 and/or NH.sup.4+ formed or the rise in the value of the pH being measured.
If the method in accordance with the invention is applied to the measurement of any kind of substrate for any kind of enzyme, the determination can be carried out in most cases with the help of a thermistor, making use of the change in temperature which almost invariably takes place in enzymatic processes.
The measurement is carried out appropriately with maintaining constant pressure and temperature in the dialyzer, and, if the analysing instrument is sensitive to the same, also at the place of actual measurement.
If a thin dialysis membrane with relatively small pores is chosen, the ultrafiltration of water will be small. Hence diffusion across the membrane will by and large be decisive. Consequently the dialysis purification is changed so insignificantly in the presence of substantial variations in pressure, that it is not necessary from a practical point of view to keep the pressure constant. It is desirable, however, that the pressure in the dialysis solution should be somewhat higher than in the surrounding complex medium, since the slight ultrafiltration which nevertheless will take place, will reduce the risk of "clotting" of high-molecular substances or cells on the fibre.
In the measurement of glucose concentrations it may be appropriate to dilute the dialysate with a weakly acid solution with the intention of lowering the pH to the pH optimum of the enzyme. If a photometric determination of the glucose concentration is desirable, another enzyme bed containing e.g. immobilized peroxidase may be used for a second enzymatic reaction between hydrogen peroxide and some substrate which gives rise to a colour reaction, e.g. ortho-toluidine. The substrate for this second enzyme stage may, depending upon circumstances, be added before or after the glucose oxidase bed.
The invention also relates to an arrangement for the realization of the above mentioned measurements in complex media. This arrangement is characterized by means for the realization of the dialysis of a small portion of the complex medium, these means comprising one or more lines for the introduction of the dialysis liquid and for the removal of the dialysate obtained, preferably via an enzyme transformation to a measuring unit.
The dialyzer appropriately consists of a fibre dialyzer with preferably one or two hollow semipermeable fibres with separate inlets through which the dialysis liquid is arranged to flow.
In certain cases it is desirable not to remove the complex medium e.g. blood, from its natural surroundings. As an alternative to the said fibre dialyzer, one or more semipermeable fibres provided with flexible tube connections may then be placed directly in the complex medium, e.g. in the blood stream, so as to bring about an intravascular dialysis.
If the membrane material and the flexible tubes are placed directly in the complex medium they should advantageously be treated in such a manner that minimum disturbance is caused in the complex material. When measuring in the blood stream it is appropriate, for example, to use heparinized material.
The arrangement in accordance with the invention appropriately comprises further means for the dilution of the dialysate with water, buffer and/or suitable reagent.
The analysis is made possible in certain cases by connecting in the arrangement in accordance with the invention between the dialyzer and a measuring unit an enzyme bed, e.g. a glass bed with immobilized enzyme for the transformation of the material that is to be measured to compounds which can be measured more readily. The desired enzymatic change can of course also be obtained in that the diluting solution after the dialyzer and/or the dialysis fluid contains the enzyme in free form.
In practice it has been found appropriate to combine the measuring unit with a computer for a direct utilization of the measuring result obtained so as to add the required components to the complex medium, e.g. blood, which is being examined.